The bombs that sprayed shrapnel into the crowd at the Boston Marathon finish line were crude devices made from pressure cookers packed with ball bearings, nails and metal shards, sources told news agencies Tuesday.

The six-litre pressure cooker bombs were hidden in bags left at ground level at two locations along the marathon route, The Associated Press reported, citing a source who had been briefed on the investigation.

The account is consistent with reports from doctors, who have described leg injuries caused by ball bearings and nails. Meanwhile, CNN reported the bombs had likely been detonated using timers rather than cellphones.

But while investigators appeared to have determined how the bombs went off, there was still no hint of why. President Barack Obama called the attack an act of terrorism but said officials could not yet say whether it was the work of domestic extremists, international terrorists or a “malevolent individual.”

The lack of sophistication of the bombs suggests it may have been a “lone wolf” acting out a grievance that remains elusive. Some experts have pointed to the symbolism of Patriots Day, which marks the anniversary of the start of the American Revolution, while others have noted that al-Qaeda had long used pressure cooker bombs.

As recently as March, al-Qaeda in the Arabian Peninsula circulated a how-to guide for making pressure cooker bombs in an online manual called the Lone Mujahedin Handbook. The introduction called for solo terrorist attacks “inside the West.”

It cited the example of Faizal Shahzad, who tried to set off pressure cooker bombs in Times Square in 2010, but failed. The Pakistani Taliban claimed responsibility for the botched attack but has denied any role in the Boston carnage.

Even though al-Qaeda has encouraged pressure cooker attacks, anyone could have used the tactic because the plans are on the Internet. The SITE Intelligence Group noted that al-Qaeda manuals had been distributed on a white supremacist Internet forum, where they were labelled “highly recommended reading.”

Pressure cookers are pots with tight-fitting lids. When filled with explosives and metal shards, the result is a significant blast. They can also be made from readily available parts. When al-Qaeda distributed instructions for making them, it titled the guide, “Make a Bomb in the Kitchen of Your Mom.”

“As a common cooking utensil the pressure cooker is often overlooked when searching vehicles, residences or merchandise crossing U.S. borders,” according to a 2004 U.S. intelligence bulletin obtained by the National Post.

U.S. authorities have been warning about the dangers of pressure cooker bombs for almost a decade, after they turned up in India, Nepal and France, where Algerian terrorists attempted to bomb a Christmas market in 2003.

“Recent reporting indicates that terrorist organizations such as al-Qaeda continue to favour the use of pressure cookers packed with explosives, a technique commonly taught in Afghan terrorist training camps,” the intelligence bulletin warned.